1. Field of the Invention
The present invention relates to a stylus for playback of high density information records, such as video discs, and also to method and device for forming such a stylus.
2. Description of the Prior Art
Generally, a video playback system of a variable capacitance form, such as CED system, includes a pickup stylus, having a thin conductive electrode, and a disc record having a relatively fine spiral groove (e.g., groove width 2.6 micrometers) formed thereon. The groove is previously formed with geometric indentations to carry information of picture and/or sound. The record substrate is provided in a shape of disc made of vinyl chloride containing carbon particles. During playback, the stylus engages the groove as the record is rotated by a supportive turntable, producing capacitive variations between the stylus electrode and the conductive record disc. Such capacitive variations are sensed to reproduce the prerecorded information.
In the system of the above described type, the use of a relatively fine record groove and the groove-engaging requirement for the pickup stylus result in a stylus end which is extremely small.
To improve the life of the stylus, a keel-tipped pickup stylus has been proposed. One example of prior art keel-tipped pickup stylus is shown in FIG. 1, which comprises a dielectric support element having a body 2, a fore-and-aft constricted terminal portion 4 and shoulders 6 and 8 joining the body 2 with the constricted terminal portion 4.
According to the prior art, the keel-tipped stylus is formed by the steps of depositing the thin layer of electrode 10 on a V-shaped stylus by way of, e.g., vacuum deposition, and then, by running the V-shaped stylus 2' on an abrasive lapping disc 12 with a deep coarse-pitched groove 14, as shown in FIG. 2, with an application of a predetermined pressure in a direction indicated by an arrow A. The lands 16 and 18 on the lapping disc 12 lap the shoulders 6 and 8 and the walls 20 and 22 of the abrasive groove 14 form the parallel side faces of the constricted terminal portion 4. Such an example of prior art keel-tipped stylus is disclosed in U.S. Pat. No. 4,104,832 to Keizer, issued Aug. 8, 1978.
According to the prior art stylus, however, there is such a disadvantage explained below. During the deposition of the electrode 10, a material for the electrode 10 may be undesirably deposited on side faces of the body 2, as shown in FIG. 1 by dots. Such a material is densely deposited in a region close to the electrode 10 and becomes sparse as the distance to the electrode 10 becomes greater. The undesirably deposited material on the side face of the body 2, particularly of the densely deposited region, senses the information of the adjacent grooves. Furthermore, if the densely deposited region is relatively long in a direction perpendicular to the electrode 10, the undesirably deposited material senses more than one piece of information at a time. The sensed information by the undesirably deposited material is introduced in the wanted signal as noise signals.
In addition to the above, according to the prior art, there are such disadvantages in the method and device for making the stylus as explained below.
Since the groove 14 of the abrasive lapping disc 12 has a very narrow groove width, such as 2 micrometers, it is necessary to present the V-shaped stylus 2' with a sharp point to properly engage the point into the groove 14. For example, the point of the V-shaped stylus 2' should be so sharpened as to have a curvature less than 1 micrometer radius R, as indicated in FIG. 2.
Also, even if the V-shaped stylus 2' is sharply pointed, it is very difficult to bring the pointed end of the V-shaped stylus 2' in engagement with the groove 14. If it is failed to bring the pointed end of the V-shaped stylus 2' in engagement with the groove 14, the pointed tip will be situated on the land 16 or 18, resulting in lapping of the pointed tip. This occurs particularly when the groove 14 of the abrasive lapping disc 12 swerves one or more micrometer in the direction indicated by an arrow B in FIG. 2, and, therefore, the possibility of landing the pointed tip on the land 16 or 18 becomes high. In this case, the pointed tip becomes undesirably flattened and, therefore, the V-shaped stylus 2' must be expelled as an unacceptable stylus, or otherwise, it must be sharpened again.
Furthermore, during the lapping, the disc 12 may undesirably makes a pitching motion in a direction perpendicular to the arrow B. Such a pitching motion may throw up and temporarily disengage the stylus 2' from the groove 14. In this case, if the stylus 2' falls on the land 16 or 18, the pointed tip will be flattened. Even if the stylus 2' falls again into the groove 14, there is such a problem that the stylus 2' may be engaged in the groove 14 in a tilted relation thereto. In this case, the lapped stylus is not acceptable.
The above disadvantages in the method and device for making the stylus results in low yield rate. According to the tests carried out by the present inventors, the yield rate of styli according to the prior art showed very poor percentage as shown in graph of FIG. 12. In the graph of FIG. 12, abscissa and ordinate represent test number and yield rate in percentage, respectively. In the graph of FIG. 12, a line C1 shows percentages of styli which have been succeeded in engagement with the groove and, a line C2 shows percentage of styli which have been determined as acceptable. As apparent from the graph, the acceptable styli is about 10 percent. Therefore, according to the prior art, it results in high manufacturing cost to obtain acceptable styli.